My standard language for Firefox and Thunderbird is Dutch.
When I use the terminal to open firefox or thunderbird with the -ProfileManager option, the resulting ProfileManager is in English.
How can I change the language to Dutch in the ProfileManager?

You mean that only profile manager is English while the main app is in Dutch?
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jasminesMay 29 '12 at 7:43

How did you get your Firefox into Dutch? Was it with an Add-On? If so, note that Add-Ons are Firefox-profile-specific, so they won't affect Profile Manager. If I download an actual Dutch-localized version of Firefox directly from Mozilla (from mozilla.org/en-US/firefox/all.html ), its ProfileManager opens up in Dutch for me.
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dholbertJun 1 '12 at 18:06

The firefox-locale-nl .deb package installs a Firefox addon and search plugin for use in profiles, rather than customizing the actual profile-independent firefox installation. See dpkg -L firefox-locale-nl -- that shows that the package just adds files in /usr/lib/firefox-addons and /usr/lib/firefox/distribution/searchplugins.
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dholbertJul 25 '12 at 23:28